Optical properties of Zn-diffused InP layers for the planar-type InGaAs/InP photodetectors *Project supported by the Key R&D Program of Jiangsu Province (No. BE2016085), the National Natural Science Foundation of China (Nos. 61674051), and the External Cooperation Program of BIC, Chinese Academy of Sciences (No. 121E32KYSB20160071). (December 2017)